Volume 93, Issue 5

Interesting analogies exist between the science of materials in small dimensions and the mechanics of biological systems. In both instances, mechanical forces and displacements are measured and the results are interpreted in the light of theoretical expectations. The approaches are, however, fundamentally different: consideration of structural diversity and global performance in biology contrasts with more quantitative evaluation of local properties and emphasis on micromechanical modeling in materials science. We report on two examples of our first efforts to combine these approaches in the investigation of biological attachment systems in insects. Nanoindentation studies were performed on the head – neck joint and the wing-locking mechanism in a beetle, and the foot – substrate adhesion in flies and geckos has been examined from the perspective of theoretical contact mechanics. By aiming at a quantitative understanding of the interrelationship between local material properties and complex structures, such studies may lead to the design of artificial attachment systems which are potentially useful in micro-technology.

A modular model for the kinetics of solid state phase transformations has been discussed, recognising three mechanisms: nucleation, growth, and impingement of growing new phase particles. For each of the three constituting mechanisms, several model descriptions can be incorporated. The choice for one specific (mathematical) description is based on microstructural information, thermodynamic data and available knowledge of the kinetics about the phase transformation concerned. The model is valid for both isothermally as well as non-isothermally conducted transformations. The model kinetic parameters are independent of the time – temperature program. In certain specific cases, the model can be simplified such that it reduces to so-called Johnson-Mehl-Avrami kinetics. From the kinetic model a method resulted to obtain the activation energies for nucleation and growth separately. The model has been applied successfully to a number of diverse transformations, involving the crystallisation of Pd 40 Cu 30 P 20 Ni 10 , the crystallisation of amorphous Mg –Cu and the austinite-ferrite transformation in Fe –Mn alloys.

Metal/ceramic interfaces between thin nickel and chromium films and the TiO 2 terminated (100) surface of SrTiO 3 were studied by various transmission electron microscopy techniques to investigate the bonding behaviour between film and substrate. For the Ni/SrTiO 3 interface, a semi-coherent interface structure between three-dimensionally grown Ni islands and the SrTiO 3 substrate was observed. From electron energy-loss spectroscopy measurements and high-resolution transmission electron microscopy studies, the formation of a two-dimensional NiO layer at the interface was concluded, which dominates the adhesion between the Ni film and the ceramic substrate. At the Cr/SrTiO 3 interface, Cr –O bonds were found, possessing an ionic-bonding contribution. Comparing the bonding characteristics of Ni and Cr on the (100) surface of SrTiO 3 , a stronger adhesion of Cr than for Ni is proposed.

We have investigated the magnetic behavior of semi-metallic CrO 2 powder samples as a function of Ar sputtering at low kinetic energy. By the use of X-ray absorption spectroscopy at the O K edge and X-ray magnetic circular dichroism at the Cr L 2,3 edges as a function of sputtering time, a strong decrease of the absolute magnetic moment has been observed corresponding to a surface reduction to a Cr 2 O 3 - like compound, deduced by chemical shifts in the X-ray absorption spectra. An unexpected change of sign of the Cr L 2,3 dichroism signal was found and interpreted by the formation of an antiparallel coupled ferromagnetic top layer. By comparison with epitaxial grown CrO 2 films a quantitative estimation of the initial Cr 2 O 3 toplayer thickness is possible.

The current achievements in magnetic transmission soft X-ray microscopy are reviewed. The magnetic contrast is given by X-ray magnetic circular dichroism (X-MCD), i. e., the dependence of the absorption coefficient of circularly polarized X-rays on the projection of the magnetization in a ferromagnetic system onto the photon propagation direction. X-MCD contrast can reach, e. g., at L 2,3 edges in transition metals large values up to 50%. Combined with a soft X-ray microscope where Fresnel zone plates as optical elements provide a lateral resolution down to 25 nm, it allows for imaging magnetic microstructures. Recording the images in varying external magnetic fields, inherent chemical specificity, a high sensitivity to thin magnetic layers due to the large contrast, and the possibility to distinguish between in-plane and out-of plane contributions allow detailed studies of static magnetization reversal processes in magnetic patterned elements and thin films. Micromagnetic simulations support the experimental findings. The broad applicability of this novel experimental technique to both fundamental and technologically relevant issues in nanomagnetism is demonstrated by selected examples. Future directions towards imaging spin dynamics on a psec time scale are briefly outlined.

Small is strong – the yield stresses of thin metall23ic films with sub-micron thickness greatly exceed bulk values. While it is clear that this effect must be due to confinement effects on dislocation processes, the details of such mechanical finite size phenomena are complicated. Our recent In situ transmission electron microscopy studies combined with discrete dislocation dynamics simulations have shed some light on this problem. One conclusion is that the behavior of dislocations near interfaces affects the plasticity in small volumes in new ways which have not been foreseen by theoretical considerations so far. Another important aspect is the discovery of unexpected slip systems, which can be seen as indirect evidence for a constrained diffusional creep mechanism in polycrystalline films. Discrete dislocation simulations have allowed the operation of dislocation sources to be simulated under various boundary conditions. Based on these studies, we propose a new view of dislocation plasticity in thin films which, while being far from complete, seems to be more consistent with experimental observations and measurements.

Fatigue, a common damage and failure mechanism in bulk metals, is largely unexplored for thin films. In the present paper, we report on the fatigue behavior of Cu films with thicknesses in the range 0.4– 3.1μm on deformable substrates. Films thicker than 1 μm seem to behave like bulk Cu and follow a Manson-Coffin relationship with a fatigue exponent and ductility of about 0.5 and 20%, respectively. For the sub-micron thick films, a clear size effect is observed: the damage morphology changes and the lifetime increases significantly. Based on a microscopical damage analysis, the following sequence for the fatigue damage evolution in the Cu films is suggested: (i) in large grains, extrusions at the film surface and voids at the interface to the substrate are formed, (ii) cracks are nucleated at these voids and grow towards the film surface, and (iii) cracks connect intergranularly to form a continuous pattern of cracks and extrusions in the film. It is argued that void nucleation is the result of the formation of vacancies due to the annihilation of edge dislocations.

Grain growth and texture development of magnetron-sputtered Ni films on thermally oxidized Si wafers were investigated as a function of sputtering and annealing atmospheres. Ni films of 1 μm thickness were sputtered in either pure Ar or an Ar–H 2 gas mixture and subsequently annealed in high vacuum or a hydrogen atmosphere at 800 °C. The films were characterized by focused ion-beam microscopy and X-ray diffraction. The Ni films had a strong {111} texture after sputtering. Films sputtered in an Ar–H 2 mixture and annealed in H 2 underwent abnormal grain growth, resulting in a {100} texture. These films had a mean grain diameter of about 50 mm. The unusual abnormal grain growth is believed to be caused by a cleaning of the Ni films during sputtering and annealing in H 2 .

The interface structure and energetic stability of Pd films adsorbed on SrTiO 3 substrates is investigated by means of ab initio electronic-structure calculations based on the local-density-functional theory. For the Pd films, (002) mono-, bi- and tri-layers are considered, and (002) halflayers as approximants for the adsorption of single Pd atoms. For the SrTiO 3 substrates with (001) surface orientation, the two possible surface terminations, with TiO 2 and SrO composition, are discriminated. It is found that Pd atoms preferably adsorb on top of O atoms of the substrate. With increasing thickness of the adsorbed film, the TiO 2 termination of the substrate is energetically advantageous for the film adsorption, and the structure of the surface atoms quickly becomes similar to the bulk crystal structure. The properties of the tri-layer film are already close to the ones of a coherent, atomically abrupt interface between bulk Pd and SrTiO 3 crystals.

It has recently been shown [1] that constrained grain boundary diffusion in a stressed polycrystalline thin film causes exponential relaxation of grain boundary traction, leading to formation of crack-like diffusion wedges via mass transport between the grain boundaries and the free surface of the film. It was assumed in [1] that surface diffusion is very fast compared to grain boundary diffusion and surface tension is very large so that the film surface remains perfectly flat during the diffusion process. In this paper, we try to relax some of these assumptions by extending the analysis of [1] to coupled grain boundary and surface diffusion. We use Mullins’ equation to describe surface evolution, with mass conservation and continuity of chemical potential strictly enforced at the junctions between grain boundaries and the free surface. The surface diffusion is treated as a matter source or sink for the grain boundary diffusion and, assuming the surface slope is small, we neglect the effect of surface slope on the normal stress and chemical potential along the grain boundary. No sliding or diffusion is allowed at the film/substrate interface. We develop singular integral equation techniques to numerically solve the integro-differential governing equations for the strongly coupled deformation and diffusion problem. The characteristic time for constrained grain boundary diffusion is found to scale with the cubic power of the film thickness and depends on an effective diffusivity combining surface diffusivity and grain boundary diffusivity. The results indicate that constrained grain boundary diffusion leads to the formation of crack-like grain boundary diffusion wedges.

The segregation of Ga at grain boundaries in polycrystalline Al was studied by analytical transmission electron microscopy. Al alloys containing 0.4, 2, and 7.8 at.% Ga were annealed, and the Ga excess at the grain boundary was determined. Segregation was detectable for volume concentrations of 2 and 7.8 at.% Ga. Embrittlement was observed only for the Al-7.8 at.% Ga alloy. The Ga excess was in the range 0.1 – 1.9 monolayers. The Gibbs segregation energies were determined for the first time ( – 110 to – 175 meV). It is concluded that in the Al –Ga system the well-known phenomenon of liquid metal embrittlement is far from thermal equilibrium.

The Stuttgart ultra-high-vacuum (UHV) diffusion bonding facility serves to produce high-quality solid solid model interfaces. An overview is given on the more recent work comprising bicrystals of metal – metal, ceramics– ceramics, and metal – ceramics combinations. Orientation errors below 0.1° have been obtained routinely with sapphire – sapphire bicrystals. The bi- and tricrystals produced serve as samples for mechanical testing and as specimens for both analytical and quantitative high-resolution electron microscopy. The close control of the interface chemistry allows for segregation studies and studies of the influence dopants have on the interface properties. UHV diffusion-bonded low-angle grain boundaries present arrays of edge or screw dislocations of just one type and thus facilitate investigations of these dislocations and their core structures.

This ongoing study aims to investigate systematically the influence of surface preparation methods on the bonding between Cu and α-Al 2 O 3 . High-spatial-resolution electron energy-loss spectroscopy and high-resolution transmission electron microscopy were used to study Cu/α-Al 2 O 3 interfaces produced by molecular beam epitaxy and diffusion bonding. Our results show that the preparation of the Al 2 O 3 surface can affect the interfacial bonding considerably. Only very weak adhesion between Cu and Al 2 O 3 could be obtained for an Al 2 O 3 substrate subjected to a chemical cleaning procedure followed by annealing. Ar + ion pre-sputtering of the Al 2 O 3 led to much stronger adhesion. Intermetallic-like interfacial bonding was observed in the samples on Al 2 O 3 substrates annealed in ultra-high vacuum after pre-sputtering; whilst in the samples on Al 2 O 3 substrates annealed in oxygen after the pre-sputtering, no significant bonding formed. High temperature and pressure applied in diffusion bonding production were not found to be determining factors for the interfacial bonding.

The deformation behavior of polycrystalline niobium with a thickness of 0.7 –2.8 mm sandwich-bonded to alumina was studied by using surface strain mapping during compressive deformation. A full-field optical strain measurement technique was employed in determining the 3-dimensional displacement and strain fields for one side surface of the bonded Nb layer, including the interfaces. The measured stress – strain curves consist of three well-defined stages of elastic deformation, yielding, and plastic deformation with hardening until fracture of alumina. Strain and displacement maps for each stage illustrate that the deformation of bonded Nb is highly influenced by the constraints from the interfaces. The yielding of Nb starts from the immediate vicinity of interfaces and extends to the whole metal layer. After yielding, strain localization maintains at the interfaces until interface debonding or fracture of alumina occurs. The strain localization at interfaces is discussed with respect to the microstructures, edge and corner effect, and the stress triaxiality near the interfaces. It is proposed that the voids at the interfaces and the stress triaxiality near an interface lead to the strain localization. The influence of the strain localization on the mechanical properties is also discussed.

A novel route is reported for the processing of nanocomposites consisting of multi-walled carbon nanotubes (MWNT) embedded in amorphous SiO 2 . SiO 2 is used for the sintered composite matrix and SiO x either for the unsintered or for the interface region with the MWNT. The material has been characterized by high-resolution transmission electron microscopy, high-resolution electron energy loss spectroscopy and thermal gravimetric analysis. Based on our observations, theoretical simulations were performed which were based on tight binding calculations. Models are proposed which acount for a stable SiO x /tube interface.

A discussion of the multiparticle diffusion-limited aggregation model for the simulation of solidification structures from a binary alloy melt under isothermal conditions is presented. The model incorporates simultaneous diffusion of all atoms in the liquid, atom attachment kinetics at the solid/liquid interface, depending on the local number and nature of bonds formed with the solid, and a surface rearrangement process that mimics the capillary (Gibbs-Thomson) effect. The influence of the local surface energy minimisation on the evolution of dendritic and regular lamellar eutectic structures and on the growth rate of a solidified layer is discussed. The role of the next-nearest neighbour interaction on the energy anisotropy of the internal (solid/solid) and external (solid/liquid) interfaces is identified with respect to the development of dendritic, seaweed and regular lamellar eutectic structures. In the case that no chemical driving force exists for the incorporation of solute (impurity) atoms in the solid, a thermodynamic condition is formulated according to which solidification of such solute atoms can yet occur.

Fe –Cr alloys containing 4, 7, 13 and 20 wt.% chromium were nitrided in a gas mixture consisting of hydrogen and ammonia at temperatures of 450, 580 and 700 °C. To investigate the nitrided samples light microscopy, scanning electron microscopy, hardness measurements, X-ray diffraction, electron probe microanalysis (EPMA), and Auger electron spectroscopy were applied. The samples with 4 and 7 wt.% Cr which were nitrided at 450 and 580 °C exhibit grains of relatively high hardness, containing finely dispersed submicroscopical chromium nitride precipitates, as well as grains of lower hardness containing discontinuously precipitated chromium nitride. The samples nitrided at 700 °C contain relatively coarse CrN particles. All samples were completely nitrided (i. e., throughout the whole sample cross-section), as confirmed by EPMA: a constant nitrogen content, independent of the occurring precipitation morphology, was observed. This nitrogen content is larger than expected on the basis of the Cr content and the solubility of N in Fe: excess N occurs. Local Cr and N enrichment was observed near pores which had developed mainly along grain boundaries. These pores are especially present in the samples which were nitrided at 700 °C. The evolution of the precipitation morphology, the presence of excess N and the local enrichment of Cr and N near pores were discussed.

Besides conventional high-temperature sintering processes for the preparation of ceramics, synthesis routes like chemical and physical vapor phase deposition or molecular beam epitaxy as well as sol-gel and polymer thermolysis carried out at decreased temperatures gain in significance. Such a trend is especially pronounced in the case of the deposition of oxide-based solids from aqueous solutions induced by organic templates. This approach is inspired by biomineralization that in general involves the formation of well-structured and complex-shaped organic/inorganic composites by the deposition of an inorganic solid on an organic matrix that consists of biomolecules like proteins. It occurs at ambient conditions with respect to temperature, pressure and atmosphere. The imitation of these processes by technical means is the deposition of thin oxide films from aqueous solutions in the presence of organic self-assembled monolayers. Besides general features of this technique like low synthesis temperatures, processing costs and equipment expenditure as well as the line-of-sight deposition suitable for coating complex shaped and/or temperature-sensitive substrates the characteristics of the films and their mechanisms of formation are discussed by way of the oxide systems TiO 2 , ZrO 2 and ZnO.